A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Although specific reference may be made to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of or about 436, 365, 355, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.
The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.
Alignment is the process of positioning the image of a specific point on the mask to a specific point on the substrate that is to be exposed. Typically one or more alignment marks, such as a small pattern, are provided on each of the substrate and the mask. A device may comprise many layers that are built up by successive exposures with intermediate processing steps. Before each exposure, alignment is performed to minimize any positional error between the new exposure and the previous ones, such error being termed overlay error.
However, some of the intermediate processing steps, such as chemical mechanical polishing (CMP), rapid thermal annealing, thick layer deposition and deep trench etching, can damage or distort the alignment marks on the substrate or bury them under opaque layers. Thus, sometimes the alignment marks are not clearly visible or not visible at all, negatively influencing the accuracy of alignment. This can cause overlay errors.
In some technologies, such as micro systems technology (MST) and micro electromechanical systems (MEMS), devices are fabricated using both sides of a substrate. This technology faces a problem with performing exposures on one side of a substrate such that they are accurately aligned with features previously exposed on the other side of the substrate. An alignment accuracy of the order of 0.5 microns or better is typically required.
New techniques are developed that allow the imaging of patterning features having even smaller patterns by using so-called immersion techniques. This technique is based on the fact that the space between the lens and the substrate is filled with a fluid, such as water. Since the refractive index of water is higher than the refractive index of air or some low pressure gas, the numerical aperture of the system is increased. This allows the system to image even smaller patterns, while using the same radiation system and projection system. More information about immersion techniques may be found in EP 1 420 298 A2, EP 1 429 188 and EP 1 420 300 A2. The presence of water may, however, influence the alignment measurements. As a result of the presence of water in the space between the lens and the substrate, it becomes difficult to perform alignment measurements on marks provided on the substrate. Furthermore, comparing such ‘wet’ measurements with previously obtained ‘dry’ alignment measurements is also rather difficult.
The conventional alignment techniques are all performed on the top side of the substrate, i.e. at the side at which the exposure is performed. This side will be referred to as the first side or the front side of the substrate. Such alignment techniques all measure the position of the substrate by measuring the position of alignment marks provided on the top side of the substrate. However, in combination with immersion techniques and with substrates fabricated from both sides, these conventional alignment techniques are difficult to perform and are less accurate.
Conventional alignment techniques typically use optical measurement techniques that measure the position of alignment marks provided on the top side of the substrate, i.e. the side of the substrate that does not face the substrate table on which the substrate is positioned. This makes it difficult to use these alignment techniques in combination with devices fabricated from both sides since, in that case, a substrate needs to be positioned such that the alignment marks face the substrate table.
In the event immersion techniques are used, the optics of the alignment system are usually positioned outside the area filled with liquid, so the optics and/or the optical signals need to be guided into the liquid. This is not an easy task and requires complicated and expensive techniques.
In order to overcome these problems, alignment techniques have been developed, that are arranged to measure the position of an alignment mark provided on the back-side of the substrate, i.e. the side of the substrate facing the substrate table and the side that is not exposed. This side will also be referred to as the second side of the substrate.
During alignment, the second side faces the substrate table that supports the substrate. In order to allow measuring the position of the alignment marks provided on the second side of the substrate, optical devices are provided in the substrate table. This technique is generally referred to as backside alignment and will be discussed in more detail below. A more extensive description can be found in U.S. Pat. No. 6,768,539.
In general, there is a need for improvements to alignment techniques, back side as well as front side, to allow a more accurate measurement of the position of a substrate located on a substrate table and to achieve better alignment results.
Throughout this specification, reference to an alignment mark being located on a particular side of the substrate includes the alignment mark being etched into a respective side of the substrate and includes having subsequent material deposited on top of the alignment mark such that it is embedded and is no longer necessarily exposed at the surface.
Although specific reference may be made to the use of the apparatus in the manufacture of ICs or MEMs, it should explicitly be understood that such an apparatus has many other possible applications. For example, the apparatus may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc.